The goal of modem communication systems is to provide their operator the best of all worlds. In severe channel conditions it is desirable to provide sophisticated coding schemes and robust transmission at the expense of achieved capacity. In moderate channel conditions it is desirable to provide maximal throughput at the expense of robustness to channel impairments. As a result, the need to provide operators the option to address such a diverse set of channels led many contemporary common standards to include a wide variety of transmission parameters.
The allowed transmission parameters may include different modulation types, constellation sizes and baud rates. Transmission power may be an important parameter, as well as the carrier frequency used. Different error correction schemes can be used such as Reed Solomon (RS) codes, Convolutional codes (such as Trellis Coded Modulation (TCM) codes and Turbo codes) and concatenated codes. Different coding rates can be used. Interleaving can be used in order to introduce time diversity. In such a case, a tradeoff between required interleaver effect and the added processing delay is usually taken into consideration. A training sequence may be used. In that case, it is desirable to properly choose the spectral characterization of the training sequence, its length, power, and constellation. All the possibilities described above may be allowed in a single communication system.
Various examples of common communication systems that allow a subset of the above possibilities are described below. The data over cable service interface specification (DOCSIS) 2.0 standard for the upstream channel is an example of a standard providing the user substantial tools to accommodate various channel impairments. The Wireless LAN 802.11 standard, with its various flavors, offers the use of several transmission parameters, such as center frequency modulation scheme (Barker/CCK/PBCC/OFDM), transmission rate, transmission power, and preamble properties. Due to the typical wireless channel parameters it is extremely important to identify the channel conditions and make a decision accordingly about what transmission parameters to use. The home phone line networking alliances (HPNA) 2.0 standard allows use of a variety of constellations (QPSK-256QAM) and different baud-rates (2 Mbaud or 4 Mbaud). This results in an achievable throughput of 4 Mbit/sec up to 32 Mbit/sec, as a function of the channel conditions. Another example of a communication system that provides numerous transmission parameters is the telephony V.34 modem. This modem provides the ability to use various constellations (from QPSK to over 1500 constellation points), six baud rates, center frequencies (two options for each baud rate), constellation shaping, control of the transmission power, the use of training sequence (TRN), and also provides tools to mitigate non-linear distortions. Dedicated signals are provided for channel characteristic analysis. These signals include training sequence (TRN), frequency comb (L1, L2), Mean Square Error (MSE) measurements and more.
After several years of ongoing debate, cable operators have selected advanced time division multiple access (A-TDMA) and synchronous code division multiple access (S-CDMA) as the upstream modulations in the DOCSIS 2.0 specification. Both of these technologies were also included in the IEEE 802.14a specification. These technologies offer cable operators the opportunity to better utilize their cable infrastructure and to generate more revenue from increased use of the cable network upstream spectrum. DOCSIS 2.0 offers operators powerful tools to mitigate common channel impairments and spectrally efficient modulations to maximize the throughput in the bandwidth-limited upstream channel. However, the many tools in DOCSIS 2.0 make the selection of transmission parameters extremely difficult in comparison to DOCSIS 1.0, with the performance of DOCSIS 2.0 systems greatly depending on the choice of these parameters.
A-TDMA is essentially an evolution of DOCSIS 1.0. It extends the physical layer of DOCSIS 1.0/1.1 with the following enhancements: 1) Additional constellations: 8-QAM, 32-QAM and 64-QAM. This allows an increase in spectral efficiency by as much as 50 percent in good quality channels and provides more increments in spectral efficiency for finer matching of data rate with existing channel SNR. 2) Additional Symbol Rate: 5.12 MB (Mega Baud). This reduces the number of receivers required at the headend for a given plant by a factor or two and improves network efficiency due to statistical multiplexing of more users in an upstream channel. 3) A byte interleaver to spread the effect of impulse and burst noise over time. 4) Improved error correction codes. DOCSIS 2.0 extends the maximum error protection ability of DOCSIS 1.0's Reed-Solomon FEC from 10 byte errors to 16 byte errors, providing greater robustness to burst and impulse noise. 5) An improved pre-equalizer for mitigating multipath distortions.
S-CDMA adds to the above enhancements a spreader that provides greater immunity to severe cases of impulse noises, and Trellis Coded Modulation, which improves performance for white noise and additional constellation of 128QAM. When in S-CDMA mode, there is no byte interleaver as described above. Instead, an S-CDMA framer introduces time (as well as code) diversity. S-CDMA calls for much stricter timing requirements in order to maintain code diversity, allowing for the elimination of guard time between data packets.
The goal of modern communication systems is to be robust while maximizing achieved capacity. To achieve this goal many standards allow the use a variety of coding schemes, constellations, error coding capabilities and other transmission parameters. The actual transmission parameters in use should be tailored to the actual channel conditions. However, in current systems, operators tend to choose overly robust transmission parameters just to be on the safe side. This results in an inefficient use of bandwidth, and a substantial decrease in capacity. In fact, standards such as DOCSIS 2.0 will only provide significant benefits to operators if and when systems make proper use of the many tools in this standard. Hence, there is a need for methods and systems that analyze channel impairments and determine the optimal transmission parameters accordingly.